The present invention pertains to the area of field emission devices and, more particularly, to ballast resistors for controlling current flow between the electron emitters and the cathode conductors of field emission devices.
It is known in the art to provide a ballast resistor in the cathode plate of a field emission device. The ballast resistor is connected to the cathode metal, which is connected to the electron emitters. The resistance of the ballast resistor is higher than that of the cathode metal. The ballast resistor is useful for controlling current flow through the cathode conductor.
It is known in the art to employ sputtered silicon for the ballast resistor. However, this prior art ballast resistor suffers from several shortcomings. First, the resistivity of the sputtered silicon is very high. In order to provide the requisite resistance, the thickness of the ballast resistor is made relatively thick, on the order of 104 angstroms. Because of the relatively thick ballast resistor, problems with step coverage can occur during subsequent deposition steps.
Secondly, the sputtered silicon has a temperature coefficient of resistance (TCR) that is relatively high. Thus, the ratio of normalized resistance over the typical specified operating temperature range (xe2x88x9240xc2x0 C. to 80xc2x0 C.) is typically within a range of 20-50. For example, ballast resistors made from sputtered silicon are known to have sheet resistances after vacuum bake within a range of 109-1012 ohms/square, which is much higher than desired. Such a large deviation in resistance can lead to deterioration of image quality at the extremes of the temperature range.
It is also known in the art to make the ballast resistor using plasma enhanced chemical vapor deposition (PECVD) of silicon, which is further doped with phosphorous or boron. The problem with PECVD silicon is its high TCR. A typical normalized resistance ratio for these prior art ballast resistors over the range of xe2x88x9240xc2x0 C. to 80xc2x0 C. is 30-100.
Furthermore, the resistance of the prior art ballast resistor can change appreciably during high-temperature process steps. For example, the sheet resistance of ballast resistors, which are made from PECVD silicon, can reach values within the range of 10 megaohm/square to 500 megaohm/square upon baking at temperatures within a range of 425-550xc2x0 C.
Accordingly, there exists a need for a field emission device having an improved ballast resistor, which exhibits reduced deviation in resistance over the standard operating temperature range and lower resistance changes during high-temperature process steps, when contrasted with prior art ballast resistors.